Case study: Fluvial Hazard Zone
2022-11-11 | CGS Admin
“The Fluvial Hazard Zone (FHZ) is defined as the area a stream has occupied in recent history, may occupy, or may physically influence as it stores and transports water, sediment, and debris.”
The CGS often collaborates with local, state, and federal agencies on projects concerning emergency preparedness and community resilience. In this case, we assisted the Colorado Water Conservation Board (CWCB), the Colorado Department of Local Affairs (DOLA), and local governments by providing technical expertise for the Colorado Hazard Mapping Program (CHAMP). The program facilitates effective long-term flood hazard reduction in Colorado. A crucial part of that process is the development of FHZ mapping protocols and debris flow hazard assessments in combination with traditional floodplain mapping. Community engagement and education on the FHZ protocols is ongoing across the state.
The initial impetus for CHAMP was the widespread devastation caused by the catastrophic flooding event along the Front Range in September 2013. Boulder and Larimer counties were the hardest hit, both in mountain communities and areas in the plains downstream. Though not the first time the state has experienced a major flooding event, 2013 galvanized opinions across the state that action had to be taken to recognize and mitigate flooding risks to the residents of the state.
“The 2013 floods perfectly illustrated the inherent shortcomings of static floodplain maps during certain events. The creation and implementation of the FHZ program goes a long way to increasing resilience in dynamic stream systems. I’m certain lives will be saved, and infrastructure will be protected when land use managers adopt this important tool.”
Kevin Houck, Chief, Watershed and Flood Protection, Colorado Water Conservation Board
Traditional FEMA Flood Insurance Rate Maps (FIRMs) are elevation-based and delineate only inundation (flooding) hazards. FHZ mapping—accomplished primarily by fluvial geomorphologists—relies on the interpretation of data describing the physical location, form, and active sediment and debris transport processes of a stream or river system. An FHZ map essentially identifies areas where a stream or river might change its path during flooding events. Important basic assumptions regarding an FHZ appraisal are that stream dimensions change during a flood event and that flows are transporting sediment and debris. The unexpected impact of these two processes on the community during a flooding event can be catastrophic and fatal. Fluvial geomorphic hazards exist within all stream corridors of Colorado.
Any regulation based on the Colorado FHZ Delineation Protocol is determined by local communities who are encouraged to explore the development of FHZ maps for the benefit of all residents. Pilot mapping of around 450 miles of stream has already been completed across the state.
“Rivers, streams, lakes, and their floodplains are major elements of healthy aquatic and riparian habitats and conveyance of flood waters. If watersheds, rivers, streams, lakes, estuaries, floodplains, and other systems are not viewed holistically as biological and geomorphologic units, serious degradation of habitat and increased flood hazards to people and human development may result.”
from ordinance text
More Resources
[Ed: See the Citations tab below the video for a link/listing of all CHAMP/FHZ publications]
Colorado Hazard Mapping & Risk MAP Portal – CHAMP StoryMap — A StoryMap describing the progress of the CHAMP program statewide.
Colorado Water Conservation Board (CWCB) – Colorado Fluvial Hazard Zone Program — Main site of the program.
Colorado Fluvial Hazard Zone Program – Resources — All FHZ publications along with a link to the Colorado Flood Threat Bulletin.
Federal Emergency Management Administration (FEMA) – National Flood Insurance Program (NFIP) — Background information on the existing federal flood insurance program.
Round River Design — Environmental consulting company and Co-Lead for the Colorado Fluvial Hazard Mapping Program.
Sand Creek Fluvial Hazards –– A StoryMap about the Sand Creek FHZ Mapping and Mitigation Plan.
Watershed Science and Design — Co-authors of the FHZ Protocol.
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Citations, Categories & Tags
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2020s, debris flow, flooding, hazards, mitigation, RockTalk, statewide, third-party, water